This article was first published in 2004.
Detonation. It’s something many car enthusiasts pretend to understand but all
too often we see the tell-tale signs that not enough people are up-to-speed.
Blown head gaskets, cracked pistons, damaged bearings – these are the sort of
terminal failures that result when your engine is severely detonating.
So what are you supposed to do to avoid becoming the next victim of
Well, before you can take the appropriate action it’s important to recognise
when detonation is most likely to occur and how it can be detected...
There are several situations where detonation may occur in normal driving.
One of the most frequent causes of detonation is driver misuse – driving in a
gear that’s too tall for a given engine rpm and load. This typically happens
when a driver doesn’t bother down-changing to exit a low speed corner, climbing
a hill in top gear, or accidentally slotting the gearbox into the wrong
In a factory high performance vehicle, detonation may also be caused by
inadequate fuel octane. Take notice of the fuel labels inside the fuel filler
flap – if premium unleaded is recommended, then that’s what you should always
strive to use. Some cars must be run on ultra high octane 98 RON fuel only – the
twin-turbo Subaru Liberty B4 is an example.
Note that the higher the fuel octane you use, the better.
Some vehicles are also very sensitive to driving conditions. If the car is
idling around in traffic for extended periods, the resulting heat-soak in the
intake manifold, induction system and under-bonnet heat can lead to detonation
when you resume driving. This is particularly common when the ECU’s intake air
temperature sensor is located near the start of the intake tract rather than in
the intake manifold.
These are the typical situations where detonation may occur in normal
driving. The next question is: how are you supposed to identify detonation during
Detonation is usually heard as a ‘ting-ting-ting’ noise that resembles
marbles rattling around in a bag. In worst-case scenarios, this ‘ting-ting-ting’
might continue for several seconds while the driver continues unaware of what’s
happening. On the other hand, it might be nothing more than a single ‘ting’.
Whatever the case, be aware that any amount of detonation is bad for your engine.
In addition to listening by ear, detonation can also be detected using
electronic devices. Detonation can be measured with various techniques but the
most common is to use a sensor mounted on the engine block. This sensor listens
for sounds transmitted through the block – the frequency of detonation is
identified amongst the myriad of other mechanical frequencies.
Several stand-alone detonation sensor kits are available in the aftermarket,
but the easier approach is to access the knock sensor input to the ECU
(where fitted). There are various LED-style meters in the aftermarket that are
designed to connect to the ECU’s knock sensor wiring.
Note that OBD-style readers can be used to identify detonation on most
late-model cars. AutoSpeed has also covered a Do-It-Yourself knock detection kit
at DIY Detonation Detector - Mk II
What is Detonation?
Engine detonation occurs when the air/fuel mix ignites within the combustion
chamber in an uncontrolled manner, instead of by the progressive action of a
moving flame front. The terms 'ping' (a light, barely observable detonation) and
'pre-detonation' (detonation caused by the ignition of the charge slightly
before the full ignition of the flame front by the spark plug) are also commonly
used. 'Knock' is another synonym.
One definition of knock is "an undesirable mode of combustion that
originates spontaneously and sporadically in the engine, producing sharp
pressure pulses associated with a vibratory movement of the charge and the
characteristic sound from which the phenomenon derives its name". If detonation
is allowed to go on for more than few seconds, the very sudden pressure changes
within the cylinder can damage the engine. In a worse case scenario, pistons,
rings and even the head itself can suffer major damage. Obviously, heavy
detonation is something to be avoided! Note also that the higher the specific
power output of the engine (ie hp per litre), the greater the likelihood of
damage if detonation occurs.
Detonation – Tolerable and
We’ve told you that any amount of detonation is bad for the engine but it is
generally accepted that there’s two types of detonation – tolerable and
Tolerable detonation occurs at low rpm, is barely audible and lasts for a
short length of time. This type of detonation can be experienced in a variety of
production cars – especially those where the manufacturer is striving to achieve
a wide spread of torque with a relatively small capacity engine.
Intolerable detonation is generally seen in the realm of modified
high-performance engines. Intolerable detonation occurs at high rpm/high load
and sometimes lasts for seconds at a time – long enough to cause massive
internal engine damage. Of course, this is the type of detonation you need to be
very much aware of...
Causes of Intolerable
As mentioned, intolerable detonation is generally limited to modified
vehicles. Let’s look at some of the common causes we’ve seen...
Adding extra ignition advance is a sure-fire way for tuners to improve
throttle response and torque but it does come with considerable risk. If
ignition occurs too early in the piston stroke, the resulting pressure wave can
ignite the air-fuel mixture in various parts of the combustion chamber, causing
Note that overly aggressive ignition timing is probably the most common cause
of detonation in modified vehicles.
Another common practice in tuning circles is to lean out the factory air-fuel
ratio to achieve a higher engine output. Well, the factory runs the engine rich
for very good reason – to control combustion temperature. By leaning the
air-fuel ratio and increasing combustion temperatures, the chance of detonation
Induction air temperature is important in all engines – naturally aspirated
and those with forced induction. In instances where an exposed air filter has
been mounted under the bonnet, the engine ingests induction air that’s perhaps
40 degrees Celsius hotter than it should be. This induction air heat is
frequently a contributing factor to detonation in modified vehicles.
In the case of a turbo or supercharged engine, the importance of induction
air temperature cannot be overstated. Induction air from a turbocharger can
reach beyond 110 degrees C – and that’s absolute murder for an engine. Proper
intercooling and/or water injection is a must.
An engine’s compression ratio also plays a major part in the occurrence of
In a naturally aspirated engine, the compression ratio is generally set up to
around 10.5:1 in order to run normal unleaded fuel. Compression ratios of around
13:1 can be accommodated by using high-octane unleaded. Be aware that if you’ve
had a hi-po motor built with a too-high compression ratio, you’ll always be
chasing your tail with band-aid fixes to detonation problems.
In the case of a forced induction engine, the static compression ratio is
usually kept down to between 7.0 and 9.5:1. This relatively low static
compression ratio is necessary to accommodate the extra air mass that is forced
into the combustion chamber by the turbo or supercharger. This forced induction
air (ie boost) increases the engine’s effective compression ratio.
The amount of boost an engine can withstand before detonation depends largely
on the static compression ratio. If the static compression ratio is relatively
high (which helps maintain off-boost driveability) the scope for lots of boost
There are several other causes of detonation in modified engines but those
listed here are generally the most common. Poor combustion chamber design used
to be a common cause of detonation, but this is generally not an issue for
modern design engines.
Note: high-octane unleaded is
recommended for any engine that is mildly modified. It’s simply not worth the
risk filling with normal unleaded.
Japanese Import Vehicles
High performance Japanese import vehicles are especially prone to engine
damage caused by detonation. This is because the engine and management system are
configured for use with the 100 RON fuel available in Japan. Import
the car to
it on local 95 RON premium unleaded – or maybe 91 RON normal unleaded – and you
have a recipe for disaster.
At minimum, we suggest using 98 RON fuel and keeping an ear out for
detonation at all times. A better solution is to take some of the steps outlined
in Part Two.
In Part Two – the final - of this series we’ll examine ways to avoid